Conventionally available are liquid crystal display devices of a VA (Vertical Alignment) mode using a vertical alignment film, offering good contrast, good working speed, and good viewing angle characteristics. In recent years, in order to increase viewing angle, there has been developed a multi-domain technique, in which a display pixel is divided into a plurality of domains and the alignment of liquid crystal is varied in these domains.
There are mainly two kinds of substrate structure which can realize such a multi-domain vertical alignment mode.
In a first substrate structure, projections are formed on portions of a base of an alignment film. According to this substrate structure, when voltage is off, liquid crystal molecules align vertically to a surface of the alignment film. However, liquid crystal molecules around the projections slightly tilt with respect to a substrate surface by being affected by a slope of the projections (hereinafter this alignment portion is referred to as a “tilt-aligned portion”).
In this substrate structure, when voltage is on, the liquid crystal in the tilt-aligned portion tilts first. Affected by these liquid crystal molecules, the other liquid crystal molecules align themselves one after another in the same direction. As a result, stable alignment is obtained in the entire pixel. That is, with the projections being a starting point, the alignment in the entire display portion is controlled.
In a second substrate structure, instead of forming projections on the TFT substrate side, electrode slits are formed in ITO (Indium Tin Oxide) pixel electrodes. In this substrate structure, when a voltage is applied, a distorted electric field (oblique electric field) is generated in the vicinity of the slit portions, enabling the alignment of the liquid crystal to be controlled with the same electric field distribution as that obtained when a structure is provided. Note that, by forming the slits and ITO pixel electrode together, the number of manufacturing steps can be reduced.
As one kind of such a conventional technique, for example, Japanese Patent No. 2947350 (registered on Jul. 2, 1999) discloses a liquid crystal display device in which a liquid crystal with negative dielectric anisotropy is interposed between a pair of first and second substrates whose surfaces have been treated with a vertical alignment process, and in which the liquid crystal aligns substantially vertically under no applied voltage, substantially horizontal when a predetermined voltage is applied, and obliquely when a voltage smaller than the predetermined voltage is applied. The first substrate includes first domain regulating means for regulating the alignment direction of the liquid crystal that tilts when a voltage smaller than the predetermined voltage is applied. The second substrate includes second domain regulating means for regulating the alignment direction of the liquid crystal that tilts when a voltage smaller than the predetermined voltage is applied. The first domain regulating means includes dielectric projections that are at least provided on electrodes of the first substrate, and that at least project into a liquid crystal layer in such a manner that a surface of the first substrate in contact with the liquid crystal is partially sloped. Under no applied voltage, the liquid crystal in the vicinity of the sloped face aligns substantially vertically to the sloped face. In a transition from a voltage OFF state to ON state, the alignment direction of the liquid crystal in the vicinity of the sloped face under no applied voltage determines the alignment direction of the surrounding liquid crystal.
Specifically, in this liquid crystal display device, as shown in FIG. 10(a), projections 104 are provided on both transparent electrodes 102 and a counter electrode 103, wherein liquid crystal 101 is interposed between the transparent electrodes 102 (pixel electrodes) and the counter electrode 103. The projections 104 provide a pre-tilt 104a for the liquid crystal 101, so as to realize divided alignment under applied voltage (ON state), as shown in FIG. 10(b).
The same effect can also be obtained when slits are formed in the transparent electrodes 102. In this case, when a voltage is applied, a distorted electric field (oblique electric field) is generated in the vicinity of the slit portions as described above, enabling the alignment of the liquid crystal to be controlled with the same electric field distribution as that obtained when the projections are provided.
In a liquid crystal display device of a multi-domain vertical alignment mode as exemplified by this conventional example, in order to widen the viewing angle in all four directions, as shown in FIG. 11(a), a counter electrode projection 202 and a pixel electrode slit 203 may be formed parallel to each other and with a tilt with respect to pixel electrodes 201. Note that, the pixel electrodes 201 are arranged side by side by sandwiching pixel aperture portions in between (not shown).
However, in this case, the liquid crystal alignment is disturbed at an edge portion X by the pixel aperture portion between the pixel electrodes 201, as can be seen in a magnified portion RG in the vicinity of a long side of the pixel electrodes 201 in FIG. 11(b).
In order to overcome such a drawback, in Japanese Publication for Unexamined Patent Application No. 229037/2002 (Tokukai 2002-229037, published on Aug. 14, 2002), pixel electrodes 211 are bent in a W-shaped zigzag fashion along counter electrode projections 212, as shown in FIG. 12. With this, as shown in FIG. 13, liquid crystal alignment 213 lines up between the counter electrode projections 212 and an edge portion M of the pixel electrodes 211.
On the other hand, as shown in FIG. 3(a) which is an explanatory diagram of the present invention, in a liquid crystal display device in which picture element electrodes 12 and source bus lines 16 are provided on substantially the same plane and the edge portions of the source bus lines 16 do not overlap thicknesswise with the picture element electrodes 12 on the both sides of the picture-element-electrode aperture portions 13, liquid crystal molecules 3a tilt in opposite directions by the oblique electric fields respectively generated on the edges of each source bus line 16 and the edges of the picture element electrodes 12.
As a result, the alignment is disturbed in the gap between the source bus line 16 and the picture element electrode 12.
To overcome such a drawback, in the above Japanese Patent No. 2947350, as shown in FIG. 14, pixel electrode aperture portions 301 formed in the transparent electrodes are bent in a zigzag fashion for example, and the pixel electrodes 302 accordingly are also formed in a zigzag fashion. A data bus line 303 is provided between the pixel electrodes 302, with a gap on both sides of the data bus line 303. Moreover, projections 304 are formed on and along the data bus line 303. Each pixel electrode 302 has a pixel electrode projection 305, and a counter electrode has counter electrode projections 306, which are provided in parallel on both sides of the picture element electrode projection 305.
With this arrangement, the projection 304 on the data bus line 303 can suppress alignment disorder caused by the data bus line 303.
However, in the conventional liquid crystal display device disclosed in Japanese Patent No. 2947350, there is the problem of increased manufacturing steps, because the projection 304 needs to be formed on the data bus line 303.
That is, in the liquid crystal display device disclosed in the above Japanese Patent No. 2947350, it is difficult to control alignment since the oblique electric field between the data bus line 303 and the edges of each pixel electrode 302 is used for alignment control. As a result, there is a problem that the alignment needs to be controlled by forming projections on the data bus line 303 of the counter electrode.
Moreover, in the conventional liquid crystal display device disclosed in the above Japanese Patent No. 2947350, passage of light through the gap D between the data bus line 303 and the pixel electrode 302 needs to be prevented. As a result, there is a problem that the aperture ratio decreases. Especially, in the case that a light-shielding layer is formed on a substrate facing the substrate on which the data bus line 303 and the pixel electrode 302 are formed, passage of light through the gap D between the data bus line 303 and the pixel electrode 302 needs to be prevented even if there is misregistration during assembly of the substrates. In this case, a large portion of the pixel electrodes 302 needs to be blocked, with the result that the aperture ratio greatly decreases.